Iron-based amorphous alloys are sought for their soft magnetic properties in applications such as the manufacturing of distribution transformer cores, pulse power cores, and other item. Iron-based alloys in this document are mainly iron alloyed with various small percentages of other metals. They are manufactured by continuous rapid solidification of a stream of molten alloy cast at speeds approaching 100 km per hour. With casting cooling rates on the order of 106° Celsius/second, the alloy atomic structure is solidified in a non-crystalline state (amorphous). With the proper atomic composition, amorphous alloy ribbons having excellent soft magnetic properties can be produced. Particularly, they offer: a high magnetic induction saturation level, herein referred as Bsat; a high permeability; a low coercive force; a low exciting power; and very low core loss. When designing distribution transformers, an alloy sheet candidate can be evaluated once it has been stacked or rolled up to form a core by considering the peak magnetic induction level B and associated core loss obtained at a peak applied AC magnetic field up to 80 A/m, herein referred as B80. Properties obtained at values above this field, even if they are good, will require excessive exciting power thus, rendering the alloy less attractive. The magnetic properties of amorphous alloys when the ribbon is stacked or rolled up to form a core can also be evaluated by considering their B80/Bsat ratio. A B80/Bsat ratio close to 1 is sought for as it is indicative of an easy magnetization. A transformer produced with a magnetic core having a high B80 will be smaller and will cost less. As a rule of thumb, an amorphous alloy, core having a B80 greater than about 1.3 Tesla is worth considering for replacing conventional grain-oriented 3% silicon steels in the manufacturing of distribution transformers. Additionally, iron-based amorphous alloys produce about one third of the core loss of silicon steels. Also, a single step continuous casting method for producing an amorphous alloy ribbon has the advantage of being simpler and cheaper when compared to casting, annealing, quenching, rolling, decarburising and coating steps involved in producing a grain-oriented silicon steel sheet and which require larger equipments and more floor space. Iron-based amorphous alloys are the cheapest of all amorphous alloys because of the relative low cost for iron, its main constituent. With a rapid solidification process, the production of amorphous alloys is however limited to a very thin ribbon of various widths. More handling is therefore required to stack the increased number of layers when building the magnetic cores. Moreover, magnetic properties of amorphous alloys are highly sensitive to internal mechanical stresses. Core loss and exciting power deteriorate in presence of random stresses in the alloy. The origin of these stresses is either residual or applied. Residual stresses appear during ribbon casting, and applied stresses are produced from external forces imposed by bending or stacking the ribbon. These stresses must therefore be removed from the ribbon when it adopts a final configuration into a core or, at least accommodated to a certain extent. Stress removal from the amorphous alloy ribbon is generally accomplished by annealing the material in a furnace at an elevated temperature for a predetermined amount of time. Also, the useful magnetic properties of iron-based amorphous alloy ribbons are obtained if, during the annealing treatment, the alloy is subjected to a uniform magnetic field or a tensile stress in direction of the ribbon longitudinal axis. Field or stress annealing reduces coercive forces and induces uniaxial magnetic anisotropy. With field annealing, the resulting magnetic anisotropy is oriented parallel to the applied field while with stress annealing, it is either parallel or perpendicular depending on the alloy composition.
Field or stress annealing will increase the B80 and B80/Bsat ratio. Furnace annealing of amorphous alloy cores with an applied magnetic field following the flux path is widely used to produce enhanced magnetic cores for distribution transformers. The ribbon is positioned within the core to have its longitudinal axis oriented following the circulating path of the induced magnetic flux.
When annealing an amorphous alloy, increasing the annealing temperature and annealing time will eventually lead to onset crystallization of its atomic structure and lost of their sought magnetic properties. Onset crystallization in amorphous alloys is a temperature-time-transformation (TTT) phenomenon. For example, the time to onset crystallization at core operating temperatures in distribution transformers must be well above the time life of the transformers. In an annealing treatment, time to onset crystallization will be influenced by the heating temperature rising rate, by the annealing temperature level and soaking time, and by the cooling temperature falling rate. High temperature heating and cooling rates combined with a short soaking time will allow use of a higher annealing temperature.
The amorphous alloy ribbon Metglas 2605SA1, from Hitachi-Metals, having a nominal chemistry Fe80B11Si9, numbers being in atomic percent, is widely used in many applications including transformers and inductors at electrical AC frequencies of 50 and 60 Hz. This alloy has a Bsat of 1.56 Tesla. When furnace annealed under an external applied magnetic field or tensile stress, the alloy will acquire an easy axis of magnetization parallel to the applied field or stress. When the ribbon is stacked or rolled up to form cores and then field annealed at 350° C. for two hours, the alloy has: a B80 of 1.49 Tesla; a B80/Bsat ratio of 0.95; and the core loss is lower than 0.27 W/kg at 60 HZ at a magnetic induction of 1.3 Tesla. These values were reported by the alloy manufacturer in an article entitled “Audible Noise From Amorphous Metal and Silicon Steel-Based Transformer Core”, in IEEE Transactions on Magnetics, vol. 44, no. 11, 4104-4106, and in an article entitled: “Advances in amorphous and nanocrystalline magnetic materials”, published in Journal of Magnetism and Magnetic Materials, vol. 304, p. 187-191, 2006. Also, U.S. Pat. No. 5,873,954 teaches that, in order to benefit of such low core loss, the 2605SA1 alloy must be annealed under an applied magnetic field for two hours at a temperature between 330° C. and 380° C. as depicted in FIG. 2a. Minimal core loss is obtained at an annealing temperature between 350° C. to 360° C. Alternatively, the patent points to some references where improvements of magnetic properties of amorphous alloys were obtained by stress-annealing. However, the sample configuration for tensile stress annealing in the mentioned references has invariably been a flat strip. Therefore, the authors of the patent consider use of stress annealing in the production of amorphous alloy core transformers impracticable. Trying to furnace anneal the Metglas-SA1 alloy above 390° C. will lead to onset crystallization of the alloy and thereby, to deterioration of the magnetic properties as reported by Hsu et al. in an article entitled: “Effect of the annealing Temperature on Magnetic property for Transformer with Amorphous Core”, Proceeding of the 2009 8th WSEAS International Conference on Instrumentation, Measurement, Circuit and Systems, page 171-175.
More recently in US patent application 2006/0180248, an iron-based amorphous alloy having a chemical composition FeaBbSicCd where 80<a<84, 8<b<18, 0<c≦5 and 0<d≦3, numbers being in atomic percent, was identified. The alloy achieves a saturation magnetic induction greater than 1.60 Tesla wherein the alloy is heat-treated to be annealed at a temperature from 300° C. to 350° C. which is lower than the temperature required for the 2605SA1 material. Included in the chemical composition, the new Metglas 2605HB1 alloy ribbon from Hitachi-Metals, having a nominal chemistry Fe81.8B15.8Si2.1C0.3, has a Bsat of 1.65 Tesla. When the ribbon is stacked or rolled up to form cores and then field annealed at 320° C. for one hour, the alloy has: a B80 of 1.55 Tesla; a B80/Bsat ratio of 0.95; and the core loss is lower than 0.24 W/kg at 60 HZ at a magnetic induction of 1.3 Tesla, which is an improvement over the commercially available SA1 material. These values were reported by the alloy manufacturer in the articles cited above.
However, furnace annealing of most iron-based amorphous magnetic alloy ribbons undesirably affects the ribbon mechanical structure. The furnace annealing treatment weakens the alloy which becomes brittle and therefore complicates ribbon handling. Furthermore, amorphous alloy cores remain highly sensitive to external stresses after annealing. Care must therefore be taken to limit these stresses in order to keep the performances within an acceptable limit. Metglas 2605SA1 and 2605HB1 are known to embrittle following conventional field furnace annealing processes and their magnetic properties are very sensitive to applied stresses.
One known method for making a distribution transformer magnetic core with an amorphous alloy ribbon was disclosed by General Electric in many patents. U.S. Pat. Nos. 4,789,849, 5,050,294, 5,093,981 and 5,310,975 disclose steps involved in the making of amorphous alloy rectangular-wound-cut core distribution transformers which address all of the particularities related to amorphous alloys mentioned above. Basically, multiple amorphous ribbons are simultaneously unrolled from supply coils, piled and then rolled up again together to produce a master coil. Then, multiple master coils are unrolled and piled to form a composite strip which is forwarded, stopped and held stationary while it is cut by shear blades into segments of progressively reduced lengths which are successively stacked in appropriate staggered positions to produce a packet of composite strips. Multiple packets are then successively wrapped in piggyback on each other on a support frame. After sufficient numbers of packets have been wrapped, a conventional silicon steel sheet is wrapped around the formed core with both ends secured together. The frame is then removed and a second silicon steel sheet is affixed against the inner packet wall within the core window to prevent the core from collapsing internally. In the following step, the core is reformed into a rectangular shape with clamps and is secured in place with supporting members and straps, after which it is batch annealed in a furnace while applying an external magnetic field for a few hours. When annealing is completed, a coating is applied on the lateral edges of the core except in the region where the joints are located to secure the laminations together, and the strap and supporting members are removed. Finally, core lacing around the electrical coils is performed by manually opening the core to form a U-shape and by sliding the core through the window of preformed rectangular electrical coils and then re-forming the core into its rectangular shape by individually closing and jointing the expanded lapped sets. Because the core laminations get brittle from the annealing process, core lacing around the electrical coils must be achieved with great care to ensure that no broken fragments find their way into the electrical coils which could lead to short circuit failures. Overall, this known process for making a rectangular-wound-cut core distribution transformer from an amorphous alloy ribbon involves a great amount of discontinuous steps, which require a lot of time and floor space. Manufacturing of such rectangular-wound-cut core distribution transformer is realized at the distribution transformer manufacturing plant. This contributes significantly to increase transformer costs.
An alternative for producing a distribution transformer core with an amorphous alloy ribbon is disclosed by Allan et al. in U.S. Pat. No. 5,566,443. In this patent, a number of electric coils are preformed, each having a portion with a shape of a sector of a circle. The preformed coils are then assembled together so that their portions combine to form a circular limb and, in order to construct the magnetic core, a continuous thin amorphous alloy ribbon is rolled up on a circular hollow mandrel located around the circular limb to produce a circular core. Before being rolled up, the amorphous alloy ribbon has been previously annealed under magnetic saturation on a second circular mandrel having the same external diameter as for the circular hollow mandrel, thus requiring a transfer of the annealed ribbon between mandrels. It is believed that the power loss associated with the cuts in the above known cut core transformer is avoided. Rolling-up-after-annealing of an amorphous alloy ribbon will certainly introduce some stresses in the roll, which will introduce some additional core loss. However, it is believed that overall introduced stresses will be sufficiently small such that a worthwhile advantage is achieved in having an amorphous alloy circular-rolled-uncut core configuration. It is also believed that with a circular-rolled-uncut core transformer, all of the above-mentioned disadvantages associated with making a rectangular-wound-cut core transformer are avoided. Furthermore, the circular core provides a shorter mean path length for the magnetic flux which reduces the core and coils sizes and weight. Although this transformer is simpler to produce than a cut core transformer, numerous discontinuous steps are still involved in the making of the core, which are: rolling up to form a core; annealing the core in a furnace under magnetic saturation; unrolling and rolling up the ribbon again to form a core around a limb of the electrical coils. Also, direct transfer of the annealed amorphous alloy ribbon will introduce unnecessary bending stresses which will cause increased core loss as the ribbon is not rolled up again at the same layer position in the circular core (the first outer layer becomes first inner layer and vice versa). This can be overcome by transferring the ribbon on an intermediate mandrel first, as taught in U.S. Pat. No. 4,906,960 by Alexandrov but, this technique adds another step in the making of the core.
Rolling-up-after-annealing of amorphous alloy circular cores as described above, although simple in appearance, remains a difficult task. The fact that the alloy becomes brittle when annealed for a significant amount of time makes it less convenient when it needs to be rolled up again around a limb of the electrical coils. Silgailis et al. in U.S. Pat. No. 4,668,309 demonstrated in Table 2 of the patent that in each attempt to unroll and roll up again an iron-based amorphous alloy ribbon of a furnace annealed circular core weighting around 50 kg at speeds up to 0.3 meter per second, the ribbon broke more than 60 times. They also claimed in the disclosed invention that annealing the cores in a molten tin bath at a higher temperature for a shorter period of time does not degrade ductility as much as from conventional furnace annealing. Silgailis et al. showed in Table 2 that circular cores weighing around 18 kg and annealed by their method could be unrolled and rolled up again at a speed of 0.76 meter per second without breaking the ribbon more than 18 times. Even if Silgailis et al. were able to significantly reduce the number of breaks with their annealing method, it remains unacceptable. Encountering just one ribbon breakage during rolling up can eject tiny fragments that will be scattered all around the assembly line and that may end up within the electrical coils, which then requires a stop in production for clean-up and a decision on whether or not the coils should be scrapped. The task becomes more difficult when rolling up of the ribbon must be performed first on an intermediate mandrel. Annealing-after-rolling of the core around the coils could overcome the problem, but this would require use of high temperature insulating materials in the coils which would render the transformer cost-prohibitive. Rolling-up-after-annealing and annealing-after-rolling of amorphous alloy ribbon cores were both considered to produce large circular cores for a heavy ion fusion (HIF) driver. A HIF core must sustain a large flux swing in an extremely short amount of time which requires use of an inter-laminar insulation in the core. Even if rolling-up-after-annealing would avoid use of a high temperature insulating material, it was considered to be impractical due to the embrittlement of the ribbon and an annealing-after-rolling of a core incorporating a high temperature resistant insulation was rather adopted as reported in articles such as: “Induction Accelerator Development for Heavy Ion Fusion”, L. L. Reginato, IEEE Proceedings of the 1993 Particle Accelerator Conference, vol. 1, p. 656-660, and: “Exciting New Coating For Amorphous Glass Pulse Cores”, R. R. Wood, IEEE 1999 12th International Pulsed Power Conference, vol. 1, p. 393-396, and: “Induction Core Alloys for Heavy-ion Inertial Fusion-energy Accelerators”, A. W. Molvik, The American Physical Society, Physical Review Special Topics—Accelerators and Beams, vol. 5, 080401, 2002. Production of circular core distribution transformers made with rolling-up-after-annealing of field furnace annealed amorphous alloy ribbon cores is impractical due to embrittlement of the alloy and therefore manufacturers are building field-furnace annealing rectangular-wound-cut core design as described above.
Thermal embrittlement of iron-based amorphous alloys induced by thermal annealing has been a recurring problem for a long time as recently stated by Kumar and al. in an article entitled: “Thermal embrittlement of Fe-based amorphous ribbons” published in 2008 in Journal of Non-Crystalline Solids, vol. 354, p. 882-888. Amorphous alloy ribbons show a ductile-to-brittle transition at a given temperature (Tdb°) below which they are brittle and above which they are ductile as reported in an article entitled: “Absence of Thermal Embrittlement in some Fe—B and Fe—Si—B Alloys”, A. R. Yavari, Materials Science and Engineering, vol. 98, p. 491-493, 1988. The fact that quenched iron-based amorphous alloy ribbons have a Tdb° lower than normal room temperature (20° C. to 25° C.) explains their ductility observed at normal room temperature. The degree of ductility at a given temperature can be estimated by observing at which bending radius the ribbon breaks or cracks, or by observing how the ribbon responds to shear cutting or tearing. An annealed ribbon having a high degree of ductility would alleviate the breaking problem and could be rolled up after annealing. Embrittlement of most iron-based amorphous alloy ribbons following thermal annealing is believed to be related to an increase of Tdb° associated to a temperature-time-transformation (TTT) dependent on the alloy composition. Keeping Tdb° below the handling temperature in order remain ductile is a target to achieve. Because the embrittlement an iron-based amorphous alloy is a TTT phenomenon during annealing, the degree of ductility of an annealed ribbon must therefore be evaluated once the core magnetic properties obtained following the annealing treatment are satisfactory or within expected results, otherwise the annealing treatment is incomplete and the degree of ductility is misleading. Shorter annealing times at higher annealing temperatures are believed to yield amorphous alloy ribbons with greater ductility. Silgailis and al. showed with their iron-based amorphous alloy ribbon cores annealed in molten tin at a higher temperature for a shorter time that brittleness could be reduced. However, there is a limit in trying to shorten the annealing time due to a limit in heat transfer capacity within the core. Higher heat transfer capacity becomes possible by heat treating a single forwarded ribbon in-line along a portion of its travelling path.
In-line annealing of amorphous alloy ribbon without thermal embrittlement has been explored. An understanding of the amorphous alloy, its annealing dynamics and its associated embrittlement has been proposed by Taub in an article entitled: “A New Method for Stress Relieving Amorphous Alloys to Improve Magnetic Properties”, published in IEEE Transactions on Magnetics, vol. Mag-20, no. 4, July 1984, p 564-569, and in U.S. Pat. No. 4,482,402. The document gives a general description on the nature of amorphous alloys, the way they are produced, the good magnetic properties of some classes of these alloys for application in distribution transformers and most importantly, discloses the necessity of stress relieving the material to benefit from its magnetic properties. According to Taub, mechanical stress relief in amorphous alloys is governed by flow and structural relaxation. Flow refers to homogeneous deformation in response to stress and structural relaxation is an atomic structure change towards an equilibrium configuration. Taub states in column 4, lines 9 to 15, “I have found that the competing material processes of flow and structural relaxation must be accounted for in order to optimize the development of soft magnetic properties in amorphous metals. Specifically, flow must be maximized and structural relaxation must be minimized. Once that state is obtained with the amorphous metal in its final shape, that state must be preserved.”
A lower viscosity at an elevated temperature in an amorphous alloy provides a low flow resistance, which allows stresses to be relieved but, on the other hand, is believed to allow structural relaxation, which increases the viscosity with time at said temperature and therefore also increases the flow resistance as the atomic structure tends to adopt an equilibrium configuration. The temperature dependence below Tg° (glass transition temperature) of both the viscosity and the viscosity increase rate with time are believed to closely follow an Arrhenius law. The structural relaxation is considered as an unavoidable consequence of stress relief annealing, which is believed to be responsible for the embrittlement of the ribbon. Therefore, prior art suggests that it is desirable to stress relieve the amorphous alloy without allowing too much structural relaxation in order to have an annealed ductile ribbon. This would correspond to keep the resulting Tdb° increase below the ribbon's handling temperature.
Taub teaches that the only way to obtain benefits of minimized structural relaxation while stress relieving the alloy is to heat as rapidly as possible to a higher annealing temperature for a shorter time and to cool the alloy sufficiently rapidly from the annealing temperature to prevent any significant additional and detrimental structural relaxation. Taub also adds in column 10, lines 8 to 13, that: “It is essential that the ribbon not to be heated until after it has reached its final configuration; otherwise, structural relaxation will commence before all the winding stresses have been applied [ . . . ]”. Structural relaxation is therefore believed to be a negative side effect of the amorphous alloy annealing process which can be minimized in rapid annealing conditions.
Taub disclosed a method and apparatus to perform in-line annealing on a forwarded ribbon of a predetermined shape. In his apparatus, a heat source, such as: heat beams; direct contact with a heating media; or resistance self-heating, heats a ribbon at a high temperature rising rate (more than 300° C./min) after it has attained its final configuration. The ribbon is then rapidly cooled (at least 100° C./min) by supplying a jet of cooling medium, such as air or an inert gas like nitrogen or streams of liquid quenchants, to the ribbon immediately after the ribbon exits the area of the heating region while still in its final configuration in order to freeze the as-annealed stress-free structure in the ribbon. The apparatus was tested on an iron-based amorphous alloy ribbon Fe81.5B14.5Si4 which was then rolled up to form a core. For ribbon feeding rates up to a maximum of 26 cm/min (0.5 cm/sec), reported results show a core loss lower than 0.28 W/kg (0.13 W/lb) and an exciting power lower than 1.45 VA/kg (0.66 VA/lb) at an AC magnetic induction of 1.4 Tesla (14 kG) and a B80/Bsat ratio (equivalent in the document to B1/B100 using Oersted instead of Tesla) greater than 0.80. The reported B80/Bsat ratio is good considering the presence of some stresses in the roll. Above 26 cm/min (0.5 cm/sec), the magnetic properties get worse. Taub also reports achieving heating rates of 500° C./min. The resulting degree of brittleness or ductility of the iron-based amorphous alloy specimens annealed with this apparatus is not quantified.
Senno et al. in U.S. Pat. No. 4,288,260 claimed an apparatus for heat-treating an amorphous alloy ribbon continuously fed under a tensile stress at a predetermined speed in the range of 1 to 50 cm/sec with its surface sliding in contact against a stationary heating body or being pressed against the surface of a heating roller by an urging roller, which can be replaced by another heating roller, to enhance the magnetic properties and remove curlings of an amorphous alloy ribbon without causing any developments of brittleness of the ribbon. In the examples 1 to 6, Senno et al. disclose results showing magnetic improvements for ribbons of given atomic compositions that were passed over a heated stationary body at feeding rates between 3.5 cm/sec (1/v˜0.28 sec/cm) and 9.1 cm/sec (1/v˜011 sec/cm). Magnetic improvements are also disclosed in example 7 for a forwarded ribbon that has been pressed against the surface of a heating roller by another roller at a slower feeding rate of 1 cm/sec (1/v˜1 sec/cm). This slower feeding rate is comprehensible as the pressed contact region between the two rollers is very small. No reference is made on using iron-based amorphous alloy compositions, as examples are shown only for cobalt-based alloys. No impacts on core loss, on exciting power, on the B80, on the B80/Bsat ratio and on the degree of brittleness of an annealed iron-based amorphous alloy ribbon are quantified through experimental results. No comparison is made with the furnace annealing method. Based on FIG. 6, the heat treatment of the ribbon passed over a stationary heating body shows a deterioration of the coercive force with increase of ribbon speed above 10 cm/sec (1/v˜0.1 sec/cm). Also, no details are disclosed on heating rates and no references are made about the cooling stage as the ribbon is simply collected on a take-up mandrel.
Gibbs discloses in UK patent application GB 2148751 a method by which a length of amorphous strip being rolled up onto a mandrel is simultaneously heated by a direct current passing through a portion of the strip that is approaching the mandrel. In this case, two spaced adjustable sliding contact electrodes (or one electrode and the mandrel) are used to supply the current. The strip is heated by joule losses from the flowing current and is either cooled before or after reaching the rolling point. However, no detailed information is disclosed on the configuration of the strip in the heat-treating and cooling zones other than optionally allowing the strip to cool on the mandrel. Gibbs only discloses reduced coercive forces measured on two non iron-based alloy samples forwarding respectively at 9 and 14 cm/sec and annealed with his method when compared to furnace annealing. There is no reference made on heating and cooling rates, to the core loss, the exciting power, the B80, the B80/Bsat ratio, or the brittleness of the annealed ribbon.
Li et al. in the U.S. Pat. No. 5,069,428 disclose an annealing method by which an amorphous ribbon slowly forwarded is self-heated by applying an AC or pulsed high current through a ribbon passing between a pair of electrodes. The circulating current through the conducting resistance of the ribbon produces joule heating. The current is passed through the ribbon while being maintained in a predetermined configuration. For a curved ribbon, the ribbon is passed over an insulated roller, preferably made of ceramic, with a pair of spring-loaded electrode rollers pressing the ribbon respectively at the entry and exit point of the ribbon on the roller. In example 1, an iron-based amorphous alloy ribbon Fe78B13Si9, alloy type 2605S2 known to have a Bsat of 1.56 Tesla, annealed at a feeding rate of 0.3 cm/sec with this process shows an improvement of the magnetic induction from 0.85 to 1.27 Tesla (8.5 to 12.7 kG) under an applied magnetic field of 160 A/m (2 Oe) compared to an as-cast specimen. The annealing embrittlement of the tested specimen has a fracture strain between 0.9 and 1 by bending test compared to 7×10 to 5×10 for furnace annealed samples. The document does not specify if the magnetic properties measurements were performed on a core or on a single ribbon. However, the resulting core loss is not clearly disclosed and no reference is made on the exciting power. The iron-based samples annealed with this method has a B80 only at about 1.0 Tesla as reported in FIG. 4 (1 Oe=80 A/m) which gives a low B80/Bsat ratio of 0.64. Also, no information is disclosed on the heating rates and on how the cooling is performed after treatment as the ribbon is simply collected onto a take-up mandrel. The authors claim a ribbon that can be annealed up to a feeding rate of 10 cm/sec with this method.
In French patent application FR 2673954, and in an article entitled “On the Optimization of Soft-Magnetic Properties of Metallic Glasses by Dynamic Current Annealing”, IEEE Transaction on Magnetics, vol. 28, no. 4 1992, p. 1911-1916, Perron et al. disclosed a joule heating apparatus similar to Li et al. to anneal an amorphous alloy ribbon in a circular shape. The ribbon is passed over an insulated fixed drum or rotating roller, preferably made of quartz or alumina, with a pair of cooled copper electrodes contacting the ribbon respectively at the touching and separating point of the ribbon on the drum or roller. In addition to Li et al.'s method, the cooled electrodes are used as a cooling means to freeze the stress relieved ribbon before it separates from the drum or roller. In example 1, an iron-based amorphous alloy ribbon, alloy type 2605S2, annealed at a feeding rate of 1 cm/sec with this process shows an improvement of the magnetization curve compared to a furnace field annealed specimen as shown in FIG. 5 of the patent. The applied magnetic field is reduced to 10 A/m compared to 14 A/m at a measured magnetic induction of 1.0 Tesla. The document does not specify if the measurements were performed on a core or on a single ribbon. Perron et al. report an average heating and cooling rates of 70 C.°/second achieved with this apparatus. They claim that, with their invention, a ribbon could be annealed at speeds near 1 cm/sec. There is no reference made to the core loss, the exciting power, the B80, the B80/Bsat ratio, or the brittleness of the annealed ribbon.
Waeckerle et al. in US patent application US2008/0196795 disclosed a ribbon annealing apparatus using an oven for heat treating a strip of an amorphous material to produce a nanocrystallized magnetic alloy of low permeability having sufficient reduced brittleness to carry out the rolling up of the strip to form cores without risk of breaking. The annealing process is carried out by forwarding the ribbon through a tunnel furnace in a flat position at a feeding rate greater or equal to 10 cm/sec and under a longitudinal tensile stress. Such heat treatment is intended for nanocrystallizing an amorphous alloy which is not sought for when the annealed amorphous ribbon must preserve its amorphous state once annealed. Also, no details are disclosed on heating rates and no references are made about the cooling stage as the ribbon is simply collected on a take-up mandrel.
If an in-line annealing apparatus was capable of annealing an iron-based amorphous alloy ribbon in a curved shape and to preserve its ductility then, the outputted ribbon could be efficiently rolled up to form a circular core around the coils of a transformer kernel such as the one disclosed by Allan et al. Using such an in-line annealing treatment would also avoid all the numerous discontinuous steps involved in making the core when using the furnace annealing method. However, this in-line annealing treatment must operate at a cost-effective ribbon feeding rate and the ribbon must acquire acceptable magnetic properties once the ribbon is rolled up to form a core. Even without considering the magnetic properties and the degree of ductility for all the in-line curved annealed amorphous ribbons of the above prior art documents, at annealing feeding rates in the 1 to 10 cm/sec range as mainly reported, a 22 cm wide and 25 μm thick ribbon (which is the widest size generally available for making conventional transformer cores) would be processed at a mass rate of 1.4 to 14 kg/hr (using the 7.2 g/cm3 material density of the Metglas 2605SA1 alloy). An average core size in a distribution transformer rated between 25 to 167 kVA weighs around 135 kg. At a mass rate of 1.4 to 14 kg/hr, this will take over 10 to 100 hours to in-line anneal the ribbon of a single transformer core. This feeding rate range is far too slow if one wants to render this process profitable. Too many annealing setups, labour and floor space would be required which increase costs. In order to be profitable, the ribbon feeding rate for a newly developed in-line annealing process must be significantly increased. Doing the treatment at a lower cost within an hour, which becomes more reasonable, requires a feeding rate above 1 m/sec, 10 to 100 times faster than the feeding rates reported above. To go beyond this rate, the heating and cooling temperature rates compared to those reported in the above methods must be greatly increased, and the annealing time must be shortened by further increasing the treatment temperature.
Performing annealing treatments in very short times on an amorphous ribbon have widely been reported in several scientific papers. Many experiments conducted on a ribbon segment have shown that the annealing time could be made much shorter. In these experiments, a ribbon specimen was generally placed between two electrodes, making contact at both ends, so that a high current pulse could be passed through the specimen using, for example, a discharge capacitor. Optionally, the experiment could be conducted in a liquid coolant for a quicker cooling. Using a suitable current density, very high heating rates can be obtained and, if followed with a rapid cooling, the annealing time can be reduced to a fraction of a second with the ribbon becoming less brittle than after conventional furnace annealing. Such experiment and results were reported by Kulik et al. in “Influence of Flash Annealing on the Magnetic Properties of a Co-based Alloy Glass”, International Journal of Rapid Solidification, 1989, Vol. 4, 287-296, and by Matyja et al. in “Rapid heating of alloy glasses”, Philosophical Magazine B, 1990, Vol. 61, No. 4, 701-713. These experiments use higher heating and cooling rates than those reported in the above prior art documents. However, the experiments were conducted on immobilized ribbon segments. Applying this method for continuously in-line annealing a forwarding ribbon is impracticable.
None of the prior art methods known to the Applicant teach a way to efficiently in-line anneal in a curved shape an iron-based amorphous alloy ribbon forwarded at a feeding rate greater than 1 m/sec and, none of them disclose circular cores made with said annealed ribbon which exhibit acceptable core loss and exciting power comparable to cores produced with conventional furnace field annealing and which have a B80 greater than about 1.3 Tesla and a ratio B80/Bsat greater than 0.80 while remaining ductile for allowing efficient rolling-up-after-annealing.